[Qemu-devel] Are "info pic" and "info irq" still of any use?

[Markus Armbruster <armbru@redhat.com>]

These commands look like bit-rotted development aids to me.

They're limited to just a few interrupt controllers.  For the most
common machine types and accelerators, they do nothing.

They complicate David Gibson's work on disentangling dependencies on
ISA.

I'm cc'ing the maintainers of all machines that can be configured in a
way that makes these commands do something.  Please speak up if you
think they provide value.



The gory details:

Only some targets provide "info pic" and "info irq", namely:

#if defined(TARGET_I386) || defined(TARGET_PPC) || defined(TARGET_MIPS) || \
    defined(TARGET_LM32) || (defined(TARGET_SPARC) && !defined(TARGET_SPARC64))

They're HMP-only.  Their documentation in full:

    info pic  -- show i8259 (PIC) state
    info irq  -- show the interrupts statistics (if available)

The first line is a lie: as we'll see, it's not just i8259, but it isn't
all PICs, either.  Not even all i8259.

Three separate implementations:

* sparc-softmmu

  This is sun4m_hmp_info_pic(), sun4m_hmp_info_irq().  Both do nothing
  when !slavio_intctl.  Set by sun4m_hw_init().  Thus, two cases:

  - The sun4m machine types

    These are SS-5, SS-10, SS-600MP, SS-20, Voyager, LX, SS-4,
    SPARCclassic, SPARCbook.  Their board initialization runs
    sun4m_hw_init().

    "info pic" prints selected registers of the "slavio_intctl" device
    model.  This isn't a i8259.

    "info irq" prints "irq statistic code not compiled" unless you
    manually enable DEBUG_IRQ_COUNT to make slavio_set_irq() count IRQs.
    Then it prints IRQ counts.

  - The other machine types

    These are leon3_generic and none.

    Both commands do nothing.

* lm32-softmmu

    This is lm32_hmp_info_pic(), lm32_hmp_info_irq().  Both do nothing
    when !pic.  Set by init() method of "lm32-pic".  Thus, two cases:

    - Machines sporting an "lm32-pic"

      These are lm32-evr, lm32-uclinux, milkymist.

      "info pic" prints selected registers of the "lm32-pic" device
      model realized last.  Fortunately, all machines have just one.
      This isn't a i8259.

      "info irq" prints its IRQ counts.

    - Other machine types

      This is just none right now.

      Both commands do nothing.

* {i386,mips,mips64,mips64el,mipsel,ppc,ppc64,ppcemb,x86_64}-softmmu

  This is hmp_info_pic(), hmp_info_irq().

  hmp_info_pic() does nothing when !isa_pic.  Set by the i8259_init()
  helper that creates a master/slave pair of the "isa-i8259" device
  model.  Cases:

  - Machines using the helper

    These are
        alpha clipper
        i386 & x86_64 pc*, isapc when accel=tcg or kernel_irqchip=off
        mips* magnum, pica64, malta, mips
        mips64el fulong2e
        ppc & ppc64 prep

    "info pic" prints selected registers of the last "isa-i8259"
    master/slave pair created with i8259_init().  I can't see a machine
    creating more than one.

    For the pc machine types, the pair of i8259s is just a piece of
    legacy crap hanging off LAPIC or I/O APIC.  The commands are thus of
    dubious utility there.

    "info irq" prints "irq statistic code not compiled" unless you
    manually enable DEBUG_IRQ_COUNT to make pic_set_irq() count IRQs.
    Then it prints IRQ counts for *all* "isa-i8259", whether they're
    created with i8259_init() or not.  Looks like all are.

  - Machines not using the helper

    These include i386 & x86_64 pc*, isapc when accel=kvm and
    kernel_irqchip=on (this is the default).

    Both commands do nothing.  Even when the machine has in fact a pair
    of i8259 implemented by device model "kvm-i8259".